Seems to me that STEEM's main value is based on what people are prepared to pay for it at any given time :-)
I know it seems like an obvious answer but if you consider that STEEM's main strength is as a way in to sites that use it such as STEEMIT.com, it will attract people who want to use those sites and earn on those sites.
So you will have people from every other social media platform (hopefully) having a presence on STEEMIT in the hope of increased revenue.
Then as more people gravitate towards STEEMIT, you may have companies and advertisers buying up STEEM to increase their influence and presence however they will hopefully be careful not to over do it as too much influence and over promoting their own services will turn people off...STEEMIT will only work if everyone is given a chance to prosper.
So in summary ..I believe the value of STEEM is derived from people's desire to be part of whatever ecosystem is powered by STEEM. Right now it's STEEMIT, in the future who knows what else might spring up.
Correct me if I'm wrong but from what I gather, the value of STEEM isn't necessarily connected with the success of Steemit. The Steemit platform could be wildly successful and yet the value of the coin could stay relatively stable. This is in contrast to other coins like ETH or EOS where a rise in adoption would see a similar rise in it's value.